The invention concerns a method for detecting contamination and/or damaging of a face that runs through a nip or nips in a calender for paper, in which method vibrations occurring in connection with the constructions of a calender are detected and processed.
The present invention is related to calenders for paper, in particular so-called soft calenders and supercalenders, in which soft-coated rolls are used, which are particularly susceptible of damage.
A particularly advantageous embodiment of the present invention is related to monitoring of the condition of soft-coated calender rolls in view of detecting their surface damage at a sufficiently early stage and, thus, in view of permitting prevention of damage in advance.
As is known from the prior art, calenders comprise two or more hard-faced and/or soft-faced calender rolls, which form a calendering nip or nips with each other, through which nip/nips the paper web to be treated is passed. In particular, rolls with soft faces, such as paper rolls or equivalent in supercalenders and rolls provided with soft coatings, in particular polymer coatings, in what is called soft calenders, are susceptible of damage. The reason for the damage is frequently contaminations, such as local fibre strings, which cause a pressure impact when they pass through the nip, which impact loads the soft coating on the calender roll and first causes its heating and, in the long run, a permanent deformation in the coating and damage. Similar deformations and, damage may also occur in metallic faces of calender rolls and in the faces of bands running through calendering nips.
In respect of the particular problems that constitute the starting point of the present invention, the following is stated. The polymer-coated rolls in prior-art calenders endure uniform loading and wear well, but they are damaged very easily is some limited, even small area is heated even to a relatively little extent, for example, to a temperature higher than its environment. Owing to the high thermal expansion coefficient and to the very poor thermal conductivity of polymers, such an even little area expands rapidly and is heated further to such high temperatures that it can be deformed. If the coating on the roll has been made of so-called thermosetting resins, on re-melting it, at the same time, loses its original properties. A heating reaction of the sort described above can be caused, for example, by a little piece of paper, fibre string or xe2x80x9cclodxe2x80x9d, or a stain separated from the coating on the paper, which adheres to the roll face and which, when entering into a calendering nip, causes a local yielding of the coating more intensive than in the environment, which heats the roll coating unevenly.
Prevention of local contamination, for example, by means of constant doctoring is, as a rule, not reasonable, neither economically nor in view of optimal quality of paper, for most of the polymer coatings that are.in use do not tolerate rubbing very well, in which case the preventive cleaning itself might wear the coating to a greater extent than the calendering proper does.
However, if contaminations could be detected early enough, for example, a cleaning doctor or some other device that cleans the roll face could be operated during short periods of time without damaging the roll coating as a result of constant or frequently repeated doctoring. In such a case, the service lives of soft-faced calender rolls could be extended to a considerable extent.
As is known from the prior art, for monitoring the condition of calender rolls, in particular of soft-faced rolls, thermometers traversing in the cross direction of the machine have been used, by whose means the temperature of the coating is monitored. In said temperature monitoring application and in corresponding other prior-art systems, problems arise from the fact that the resilient roll coating, whose temperature is monitored, is, as a rule, at least to some extent electrically insulating. Thus, in the partly rubbing contact between the web and the coating, quite high charges of static electricity arise in the faces of the roll coating and of the relatively dry paper web. These charges tend to be discharged along the available routes with the lowest resistance. A thermographic camera must often be installed so that it projects from its support construction, in which case said static electric charges find exactly the thermographic camera as the easiest route of discharge, in which connection the sensitive electronic system of the thermographic camera is subjected to quite high voltages, and it must be protected specifically against such voltages.
Even if monitoring of the temperature of the face of a calender roll usually permits detecting of a local raised temperature resulting from an excessive load applied to the coating or from a local inner non-homogeneity at a sufficiently early stage, this requires installation of quite heavy, expensive and spacious equipment in the vicinity of the roll to be monitored. In particular, congestion of space causes great difficulties in connection with multi-roll treatment devices, at which every device that is not included in the web treatment process proper makes the maintenance and servicing of the device more difficult.
As is also known from the prior art, DE 43 40 700 discloses a method for detecting contamination or damages in a calender roll by detection of vibrations with vibration sensors. Damaged rolls are replaced in accordance with detected information to stop further damage in a papermaking process.
The object of the present invention is to provide a method by whose means contamination and/or damage in faces of rolls that form a calendering nip or nips and/or in faces of bands that run through nips can be monitored efficiently and by means of relatively simple devices that require little space.
It is a further object of the invention to provide a method in which the operations to be controlled based on the monitoring of the face running through the nip can be arranged to be so quick that, as a rule, permanent substantial damage does not have time to be produced in the face concerned before cleaning operations or equivalent have been started.
It is a further object of the invention to provide a method by means, of whose application, when necessary, the cleaning of the face running through the nip can be applied sufficiently precisely expressly to the area that has been noticed to be contaminated, whereby economies can be obtained in respect of the medium and the energy used for the cleaning. In some cases, owing to the invention, the cleaning equipment can be dimensioned so that its capacity is lower than in the prior art.
In view of achieving the, objectives stated above and those that will come out later, the method in accordance with the invention is mainly characterized in that said vibrations are detected by means of at least one vibration detector fitted in connection with the bearing supports of calender rolls or in connection with constructions related to said bearing supports, by means of which vibration detector(s) the calender roll(s) is/are identified from which the vibration derives.
In a preferred embodiment of the invention, on the bearing housings of the calender rolls or at least on a part of said bearing housings and/or on the roll frames, at least one acceleration detector which measures vibration of the calender roll or a strain gauge detector which measures deformations arising from vibration or other, equivalent detectors is/are fitted. The measurement signals from these detectors.are monitored and analyzed so that the portion of the measurement signal that arises from rotation and unbalance of the rolls is xe2x80x9cfilteredxe2x80x9d off, and the development of the portion that arises from the geometry of the roll face is monitored. When the signal to be monitored reaches a predetermined level or when its spectrum is placed within the measurement window, the control system reports this further, and action can be taken either to replace the roll or to eliminate the source of disturbance.
By means of the system of the present invention, vibrations of the faces of calender rolls to be monitored are, in a way, xe2x80x9clistened toxe2x80x9d and, to the necessary extent, the site of origin of the vibrations is located so that efficient action can be taken sufficiently quickly in order to eliminate the cause of the vibration and/or to prevent further damage.
In the invention, the locating of the source of disturbance can also be carried out so that the rotation of all or several of the rolls to be monitored is observed and that vibration is measured from any suitable point whatsoever, for example from the frame of the machine. By comparing the measured vibration signal, preferably the cycle time of its repetition, with the measured speeds of rotation of the different rolls, it is possible to determine the source of the disturbance and to take action in order to eliminate the disturbance either by cleaning the roll or by replacing the roll before it is damaged.
By means of a monitoring system that applies the present invention, it is possible to create such information data bases by whose means the optimal moment of replacement of the rolls can be determined when the seriousness of the damage that has resulted in replacement of a roll is classified and stored in the memory in connection with replacement of each roll. In such a case, by means of history data, it is possible to identify the seriousness of a disturbance and to take action at the correct time.
By means of the present invention, it is possible to eliminate the problems discussed above by abandoning direct measurement of the surface temperature of the roll, by indirectly monitoring local impurities on rolls and changes in geometry on the basis of vibrations produced by them.
Advantages of the present invention, as compared with the prior-art procedure, are the following:
In the most advantageous embodiment of the invention, it is possible to locate the soft-faced calender roll, in particular a polymer-coated roll, that has been contaminated or deformed, so that the roll can be cleaned, for example doctored, before it has been damaged and become unusable, or, if necessary, a damaged roll can be replaced so that further damage or longer interruptions of production can be avoided.
The invention permits monitoring of the calender rolls so that, in the area of the roll frame, it is unnecessary to construct a measurement equipment provided with a separate support construction, in which case considerable economies of space are obtained.
The invention also permits locating of a calender roll that causes a disturbance or, if necessary, of an area of said roll by means of relatively simple devices and algorithms.
The invention can be connected as a part of the general system of monitoring of the condition of the calender, and the invention can utilize the assemblies of detectors already installed in the area of bearings for the purpose of monitoring the condition of bearings, or, alternatively, the detectors installed in order to carry out the present invention in view of monitoring the condition of the roll face can also be utilized for predicting and/or monitoring of bearing damage.
The invention is not confined to monitoring of disturbance arising from changes in temperature related to local surface temperatures on the calender rolls alone, but the system of the present invention already reacts at such an earlier stage in which a contamination has adhered to the roll face which would later additionally cause a locally raised temperature.
Further, it can be considered to be an advantage of the invention as compared with a thermographic camera that the system of the invention can, if necessary, be configured so that it constantly monitors the portion of the roll across the entire width of the treatment nip and locates the cause of disturbance even both in the axial direction of the roll and in a vertical cross section of the roll in the direction of its circumference.
When the monitoring system of the invention is used in a calender with several nips, for example in a supercalender, all the rolls in the calender, or at least the rolls that are particularly susceptible of surface damage, in particular soft-faced rolls, are provided with a detector system in accordance with the invention, in which case the source of unusual vibrations can be located exactly at a contaminated and/or damaged roll so that it is possible to start eliminating the contaminations or to replace the damaged part so that further damage and production losses are avoided.